1. Field
Circuit patterning.
2. Background
Patterning is the series of steps that results in the removal of selected portions of surface layers added on a substrate such as a wafer. Patterning defines the surface parts of devices that make-up a circuit. One goal of patterning is to define in or on the wafer surface, the parts of the device or circuit in the exact dimensions (feature size) required by the circuit design and to locate the parts in their proper location on a wafer surface. Generally speaking, patterning sets the critical dimension of devices of a circuit.
Generally, patterning is accomplished through photolithography techniques. In general, photolithography is a multi-step pattern transfer process whereby a pattern contained on a reticle or photomask is transferred onto the surface of a wafer through a lithographic imaging step, including the development of a light sensitive material (e.g., photoresist) on the wafer. In general, the smallest feature printable by the imaging system is proportional to the following quantity:
  λ  NAwhere λ is the wavelength of light used in imaging the mask onto the wafer and NA is the numerical aperture of the projection optics.
One goal of circuit designers is to reduce the feature size (the critical dimension) of devices of a circuit, i.e., reduce the smallest feature patternable. A reduction in wavelength of light used in patterning will reduce the smallest feature patternable as will an increase in the numerical aperture of the lens. Unfortunately, an increase in the numerical aperture of the lens tends to be quite expensive. Thus, the recent trend has been to reduce the wavelength. Currently, wavelengths of light used in patterning integrated circuits are 248 nanometers (nm) or less for a critical dimension on the order of 130 nm. As the critical dimension approaches 100 nanometers or less, the wavelength will be reduced to under 200 nanometers, and will eventually lie in the extreme ultraviolet (EUV) region, 10 to 100 nm. In fact, for certain applications, shorter wavelength photons, i.e. less than 10 nm (xray) or charged particles (electrons, ions) may be employed.
In the general course of patterning, the image of a reticle or photomask is projected onto a wafer by an imaging system. Typically, the imaging system is refractive and is composed of lenses fabricated out of glass or quartz. EUV radiation, however does not pass through quartz or glass. Thus EUV imaging relies on reflective optics. EUV radiation is reflected off a mirror onto a wafer in a photolithographic imaging step. In charged particle lithography, electric and or magnetic fields are used in the place of reflective or refractive materials to direct and focus and imaging radiation.
In one photoresist composition suitable for use in patterning circuit devices using EUV, the composition includes a photoacid generator (PAG) in a polymer having a deprotection unit. Upon irradiation, the PAG generates an acid that, in a subsequent post-exposure-bake process catalyzes a deprotection reaction, a cross-linking reaction, or other reaction that affects the solubility of the photoresist. The result is that the solubility of the resist composition in a developer is differentiated between the exposed and unexposed regions and as a result either positive or negative images are achieved.
Electron absorption is recognized as one recognized mechanism by which EUV photoresist films receive patterning signals to form lithographic features.
These electrons are the product of the original aerial image from EUV photons impinging upon and ionizing atoms in the photoresist. The distance an electron travels from the point of ionization within the film or at the uppermost part of the substrate (e.g. the substrate surface) to a PAG is a propagation length. Typically too, PAGs such as triphenyl sulfoniums are relatively small species (e.g., volume on the order of about one cubic nanometer) and include relatively electron transparent moieties (e.g. hydrocarbons and sulfur) and therefore have a relatively limited electron capture cross-section. Thus, the uncontrolled propagation length of electrons within the photoresist blurs the original aerial image by a finite amount, limiting the resolution of the film and contributing to feature roughness.